As well known, a CDMA (Code Division Multiple Access) method, serving as one type of multiplex (multiple access) radio communication method, shows concealment and interference resistance, and is being used for diverse communication systems as a communication mode capable of achieving effective use of radio frequencies. In the recent years, this CDMA method is also being applied to mobile communication systems because of the solution to the problem involving “degradation of transmission quality stemming from considerable positional variation (or difference) of an individual terminal station existing a radio zone established by a radio base station” (referred to generally as a far-and-near problem).
Meanwhile, a study has been promoted for realizing a receiver (“software radio equipment”) which deals with a plurality of modulation modes by downloading software handling the plurality of modulation modes to the minimum hardware according to each alteration of the modulation mode, and for example, it can be considered used as a regulatory apparatus on illegal radio equipment.
In addition, in the recent years, there exists an increasing need for the detection accuracy of a received signal strength indicator (RSSI) in these radio equipment (terminal stations). For example, for the solution to the foregoing “far-and-near problem”, in general there is a need for the received signal strength indicator detection accuracy to be below ±1 dB, and also for the “software radio equipment”, considering the estimation of the presence or absence of transmitted electric wave from an illegal radio equipment on the basis of a received signal strength indicator, a requirement exists for the improvement of the received signal strength indicator detection accuracy (usually, below ±several dB).
As well known, the aforesaid received signal strength indicator is detected by a received signal strength indicator (RSSI) detecting section placed in the interior of a receive system (receiver) of a radio equipment, and for example, this RSSI detecting section is frequently constructed as a logarithmic amplifier (LOG AMP) in which a plurality of operational amplifiers 101 are connected in a multistage fashion as shown in FIG. 11.
Moreover, in the RSSI detecting section (logarithmic amplifier) 100 shown in FIG. 11, the number of operational amplifiers 101 saturated increases (the number of operational amplifiers 101 operating linearly decreases) as the strength of an inputted (received) electric field becomes higher. Conversely, the number of operational amplifiers 101 saturated decreases (the number of operational amplifiers 101 operating linearly increases) with a lower inputted field strength.
Accordingly, for example, as FIG. 11 shows, if the values obtained by normalizing the input voltages to the respective operational amplifiers 101 by means of operational amplifiers 102 (for example, the output voltage of the operational amplifier 101 saturated is set at “1”) are added to each other in an adder (Σ) 103, an output voltage value is obtainable according to the inputted field strength. FIG. 12 shows an example of an input field-output voltage characteristic of the logarithmic amplifier 100.
Furthermore, the received signal strength indicator of radio equipment is estimated (detected) on the basis of the output voltage value of the logarithmic amplifier 100 thus obtained, and in a conventional art, the output voltage value is converted into a received signal strength indicator detection value through the use of a memory in which received signal strength indicator detection (estimated) values to the output voltage values are recorded in the form of a table. For example, the logarithmic amplifier 100 has the input field-output voltage characteristic shown in FIG. 12, and when digital values obtained by the A/D conversion of the output voltage values are set as addresses of the aforesaid memory, the data to be recorded in this memory are taken as shown in the following table 1.
TABLE 1Example of Memory DataOutput Voltage (v)0.4 to 0.5to 0.6to 0.7to 0.8to 0.9to 1.0to 1.1to 1.2to 1.3to 1.4to 1.5Address (hex)28323C46505A646E78828CData (dBm)−70−65−60−55−50−45−40−35−30−25−20Data (hex)46413C37322D28231E1914
For example, when the output voltage value of the logarithmic amplifier 100 assumes 0.84 V, as the table 1 shows, the memory read address is 50 (hex), and in this case, the received signal strength indicator is estimated to be −50 dBm. In like manner, for example, when the output of the logarithmic amplifier 100 is 1.26 V, the memory read address is 78 (hex), and the received signal strength indicator is estimated at −30 dBm.
Naturally, the direct use of the values recorded in advance in the memory causes the degradation of accuracy of the received signal strength indicator. For this reason, for the compensation of this degradation, an interpolation between data is made using linear approximation or the like. In the above-mentioned examples, the use of this linear approximation provides 0.84 V→53 dBm and 1.26 V→32 dBm, and the improvement of the accuracy is achievable.
However, in the case of the detection of the received signal strength indicator by the logarithmic amplifier 100 mentioned above, since the logarithmic amplifier 100 usually has an input field-output voltage characteristic depending upon an input waveform, an offset occurs in the output voltage value according to the type of waveform of a received signal. For example, in the configuration shown in FIG. 11, in a case in which the input (received) signal is a DC signal or a rectangular wave signal, when the input signal (input voltage) is taken as Vin and the output signal (output voltage) is taken as Vout, the transfer function of the logarithmic amplifier is represented by the following equation.Vout=Vy·LOG(Vin/Vx)  (1)where Vx depicts an intercept voltage and Vy denotes a slope voltage, with both being a fixed voltage which determines the scaling of the logarithmic amplifier 100.
Thus, in a case in which the input signal is other than the DC signal and the rectangular wave signal, these signals serve as reference waves, and as the following table 2 shows, an offset (error) occurs in the output voltage value of the logarithmic amplifier 100.
TABLE 2Output Voltage Offset Values to waveformsPeak/EffectiveInterceptError (dB) to DCInput WaveformValue rmsFactorSignalDC Signaleither10Rectangular Waveeither10Sine Wavepeak2−6.02Sine Waverms√ 2−3.01Triangular Wavepeak2.718(e)−8.68Triangular Waverms1.569(e/√ 3)−3.91Gaussian Noiserms1.887−5.52GSM *1 Waverms1.507−3.56PDC *2 Waverms1.511−3.59COMA *3 Waverms2.128−6.56In this table 2,*1 Global System for Mobile Communications (Modulation Mode: GMSK (Gaussian filtered Minimum Shift Keying)*2 Personal Digital Cellular (Modulation mode: π/4 Shift QPSK)*3 Code Division Multiple Access (Modulation Mode: QPSK for Both Modulation and Spread)
Accordingly, for example, in a CDMA system, since the received waveform in a terminal station with respect to the same carrier varies with the number of multiplexes (the number of Spread codes), an offset takes place in the output voltage value of the logarithmic amplifier 100, which causes an error in detection of the received signal strength indicator. For example, in the case of an N (Narrow band)-CDMA system, as FIG. 13 shows, since shifting occurs on an input field-output voltage characteristic of the logarithmic amplifier 100 according to the aforesaid number of multiplexes, even if the received (inputted) electric field is the same, an error up to approximately 4 dB occurs in a received signal strength indicator between when the number of multiplexes is “1” (single code) and when it is “60”.
In the case of the CDMA system, this received signal strength indicator detection error in a terminal station leads to a decrease in the number of terminal stations a base station accommodates. In the CDMA system, this is because, upon the power-on of the terminal station, its own (terminal station) sending power value is determined on the basis of the performance of the present received wave and the received signal strength indicator detection value at that time is communicated to the base station so that a so-called “open loop processing” is conducted on the base station side to determine a sending power value to the terminal station according to the receive performance (received signal strength indicator detection value or the like) of a transmitted wave from the terminal station.
That is, in this “open loop processing”, when the received signal strength indicator detection value shows an error in the terminal station, both the upstream sending power value from the terminal station to the base station and downstream sending power value from the base station to the terminal station can be determined to a sending power value larger than the sending power value needed actually, and in such a case, the interference component among the terminal stations increases, which limits the number of terminal stations communicable with the base station at the same time and at the same frequency. Moreover, naturally, this causes increased power consumption.
On the other hand, also in a terminal (multimode terminal) of a multimode handling system such as a TACS+N−CDMA system or the aforesaid “software radio equipment”, a difference in transmission waveform from a base station to a terminal station originating from a difference in mode, such as a TACS/CDMA mode, or a difference in modulation mode produces a similar offset in the output voltage value of the logarithmic amplifier 100, which lowers the detection accuracy of the received signal strength indicator.
For preventing the lowering of the received signal strength indicator detection accuracy, for example, it is considered that the above-mentioned received signal strength indicator estimation memory is prepared for each mode or each modulation mode, but this consideration contributes to the enlargement of circuit scale.
The present invention has been developed in view of the above-mentioned problems, and it is an object of the invention to provide a radio communication system, received signal strength indicator compensating method for use in a radio communication system, and base station and terminal station for a radio communication system, capable of enhancing the detection accuracy of a received signal strength indicator by compensating for an error in detection of the received signal strength indicator in a terminal station.